This invention relates to an X-ray receptor and, more particularly, to a solid-state X-ray receptor, a method of making that receptor, and the manner in which the X-ray receptor is used to produce electrical signals representing an X-ray image. Although the invention finds particular application in the field of medical diagnostics, it also is applicable to the general field of X-ray imaging, including mechanical structural analysis, article detection and sensing, and the like.
X-ray imaging has long been used for medical diagnostics. Typically, a patient is exposed to X-radiation which is transmitted from a suitable source through the patient to impinge upon an X-ray sensitive film or plate. The resultant X-ray image is developed, analogous to the development of a photographic image, from which a suitably trained physician may diagnose the the patient's condition. While this technique has been generally accepted in the health care industry, it suffers from notable disadvantages and deficiencies. For example, a delay of several minutes to several hours in the development of the X-ray image may occur. As a result, prompt analysis and diagnosis, although desirable, may not be possible. Furthermore, when X-ray image development is carried out in a large facility, such as a large metropolitan hospital, the exposed X-ray film or plate may be misplaced or lost and, notwithstanding safeguards that may be employed, the developed image may be falsely identified.
Another disadvantage associated with conventional X-ray imaging techniques is the delay and inconvenience that may result if repeated or additional images are needed. For example, if one X-ray image is not satisfactory, a substitute may be needed. However, the adequacy of the original image might not be ascertained until long after that image has been made and developed. In a large facility, such as mentioned above, a patient may be recalled days later or, in the alternative, the patient may be required to spend many idle hours at that facility awaiting the determination of whether a substitute X-ray image is needed. Similarly, if the technician or physician determines that an alternative X-ray image is needed from, for example, a different angle or direction, the patient must be recalled or must remain at the facility until this determination is made. To minimize the inconvenience of recalling or retaining the patient, it is not uncommon for X-ray technicians to take multiple images from several different directions or angles of the same patient with the expectation that at least one of such images will be satisfactory. However, multiple exposures results in subjecting the patient to higher dosages of X-radiation, which is not desirable.
A still further problem associated with present-day X-ray imaging techniques is the requirement of maintaining a relatively large inventory of X-ray film or plates for X-ray exposure and an even larger library of exposed film. In addition to the expense attending such a large inventory, it also is necessary to provide high quality control to assure that such film or plates are of good quality. In addition, a suitable laboratory or other facility is needed to process the exposed X-ray film or plates. Additionally, a considerable amount of time and money are expended in maintaining the aforementioned film library and in manually handling the film each time one or more exposed images is recalled from and then returned to the library.
There has, therefore, been a long-felt need for a so-called "real time" X-ray receptor which produces electrical signals directly in response to impinging X-radiation, which signals are processed to produce a viewable image on, for example, a cathode ray tube. Fluoroscopic apparatus has been known for some time; but this is less than satisfactory in many applications because of, for example, the limited dynamic range (perceived as contrast) of the image intensifier normally used with such apparatus, and the further degeneration of images that are displayed by conventional television techniques. With the advent of sophisticated digital processing, it is desirable to produce electrical signals representing an X-ray image, as opposed to a mere fluoroscopic image which is produced directly from X-radiation. Such image-representing signals, when converted into the proper format for digital processing (i.e. when they are "digitized"), may be suitably processed, image-enhanced, stored and recalled (e. g. magnetic or optic recording). The digitized image-representing signals advantageously may be supplied to and used by digital micro-computers whose capability of performing algorithmic functions on the image information substantially enhances and exploits the raw data present in the image-representing signals.
It has been proposed heretofore to provide X-ray image pickup tubes of a type similar to television or video pickup tubes. Such tubes are, however, difficult and expensive to manufacture. Moreover, such X-ray imaging tubes must be handled with great care to avoid damage. Furthermore, such tubes generally require relatively high electrical voltages and currents for proper operation. Still further, it is not practical to construct an X-ray image pickup tube having a pickup screen, or surface, that is comparable in size to that of conventional X-ray film or plates. As a typical example, such film or plates may be on the order of 14.times.17 inches; and it is quite difficult to produce a pickup tube whose screen is of this size.
The aforementioned problems associated with X-ray image pickup tubes are overcome by the present invention which provides a solid-state X-ray receptor that produces signals readily adapted for digital processing, storage, and enhancement. It is believed that, heretofore, solid-state X-ray imaging devices have not been practical due, in part, to the inability of utilizing satisfactory solid-state X-ray imaging elements. Although semiconductor photodetecting elements have long been known, such elements generally are not acceptable for use in X-radiation environments. For example, silicon photodiodes are soon rendered inoperative or destroyed when exposed to X-rays. Other semiconductor materials are responsive to radiant energy whose wavelengths lie in the ultraviolet, visible and infrared bands, but not to energy having the much smaller wavelengths that constitute the X-ray band. Reference is made herein to Chapter 6 of the text "Integrated Circuits and Semiconductor Devices" by Deboo et al., McGraw-Hill Book Company (1971) for a discussion of such photodetectors.
Another difficulty that is associated with solid-state X-ray receptors is that, assuming a suitable semiconductor X-ray responsive material is known, a satisfactory imaging device using this material must be formed of several semiconductor elements, each used to produce a "picture element" or "pixel", representing a discrete small area of the X-ray image. The necessary array of elements must be suitably supported in a plane so as to produce the corresponding pixel signals. Such elements must be closely spaced both in the horizontal and vertical directions. As a numerical example, a suitable array should be formed of 1024.times.1024 such elements. Heretofore, it has not been thought practical to support over one million semiconductor elements in proper orientation, to receive the signals produced by each element and to process those signals so as to result in a suitable viewable image. Rather, a single X-ray detector (whose construction is not readily ascertainable) has been moved from point-to-point in order to produce a composite X-ray image (see, for example, U.S. Pat. No. 4,128,877), or a linear array of detectors (whose construction also is unknown) has been provided in a movable yoke that also is moved in order to reconstruct a composite image (see, for example, U.S. Pat. Nos. 4,136,388 and 4,259,721). The requirement of moving a single detector or a linear array of detectors is time-consuming, often resulting in movement by the patient, which reduces the quality of the X-ray image and information contained therein, and is further subject to displacement errors inherent in the movement of the detectors from point-to-point.